Sculptors wishing to make a bronze statue must first sculpt their model in clay. An armature to support the clay is built roughly to the shape of the envisioned figure and secured to a base. Armatures can be made of wire, wood, Styrofoam or any other rigid material. Water-based or oil-based clay is added over the armature to flesh out the shape and add detail to the sculpture.
The original is taken to a trained mold maker. The mold maker divides the model into segments with large flat shims embedded into the clay. This step determines how many segments are needed to cast the model. A two-part mold is the simplest mold, but more complex multi-part molds are often required. Rubber is painted on to the model and out on to the shims. The rubber mold is then covered with a thick layer of plaster. The rigid plaster mother mold supports the soft rubber mold. The mold is pulled off the original clay model. The model is usually damaged and returned to the artist so she may recover the clay and armature for future use.
The mold is turned upside down and hot wax is poured into it and gently slushed around. The wax coats the inside of the mold and any additional liquid wax is poured out. The process of slushing and pouring out the remaining wax is repeated several times until a shell of hard wax is created inside the mold. The wax is removed from the mold and reassembled; the seams are cleaned and defects are repaired. The wax model should be an exact duplicate of the original. This is called chasing the wax. The artist is called to approve the wax before proceeding to the next step. After the wax is approved, several wax models may be made.
Another mold is made in preparation for pouring the bronze. The wax model is cut into segments and dipped into a silica slurry which hardens around the wax forming the waste mold. This is also called investing the wax. The wax is then burned out of the mold to leave a void for the molten bronze. Bronze is melted in a furnace to approximately 2000 degrees Fahrenheit. The pot, called a crucible, is lifted out of the furnace with a clamp attached to long poles by two individuals. The waste molds, having been positioned in a sand pit, are filled with molten bronze. After the bronze is poured, the waste mold is chipped off to remove the cooled bronze segments.
The bronze pieces are reassembled and welded together. The sculpture is finished by a process called chasing, by which the seams are smoothed and flaws are fixed. The final step in chasing is to lightly sandblast the entire piece to prepare it for patination. Patination is the final coating that gives the bronze its finish and color. The person who applies the patina is a highly skilled craftsman who has training in the chemical process and safety procedures of patination. Many patinas are dangerous and some are deadly if inhaled. A final coat of wax is added to prevent oxidization. No other care is necessary and would damage the patina if caustic cleaners were used.